A typical context of application in which there arise conditions such as the ones outlined above is the production of disposable absorbent sanitary articles with use of web materials that have characteristics of extensibility.
By “extensibility” (or “elasticity”) is meant in general the fact that the material may be subjected to tensile force and extended (lengthened), even to a significant extent, for example 100% of its initial dimension at rest, i.e., with a lengthening of 100%, so that the material is brought to a length equal to twice its length at rest. Very frequently, the materials to be elasticated in sanitary products are materials made up of a number of layers, for example two layers or sheets of non-woven fabric, set and anchored between which is a sheet or web of elastic material.
The extensibility of the web can on the other hand create difficulties during treatment of the material, above all if there is taken into account the fact that the web wound on the reel does not usually present physico-mechanical characteristics (for example the modulus of elasticity or in general the characteristics that determine, given one and the same tensile force or “pull” applied, lengthening and/or “neckdown”, with the corresponding values of deformation) that are absolutely constant; said characteristics are in fact subject to a certain degree of variability also within one and the same reel.
In a possible application, the web on the reel is a web of polyethylene (pre)printed on which are patterns and decorations. The printed web is fed through various movement and tensioning elements (such as rollers, nip points, transmission bars or “feathers”) until it reaches a treatment or processing station functioning in a cyclic way, which applies on the web further elements such as, for example, absorbent layers, labels, and the like. Said elements are applied on the web at regular intervals apart in space and, from this standpoint, the web can be viewed as being made up of successive stretches, each of which corresponds to a single article that is to be obtained from the web. It is therefore important that the stretches of web should be fed to the processing station in a given phase relationship with respect to the operating cycle of the treatment station. In this way, it is ensured that the elements applied on the web are “in phase” with the patterns and/or decorations present on the web in order to prevent the latter from being “off-centre” or “misaligned” with respect to the individual article produced.
In general, it is possible to control advance of the web to the treatment station in such a way that feeding of the web to the treatment station is constant.
This result can be pursued by applying, for example, a constant-mass-flow model. However, the variability of the characteristics of the web, particularly the variations in elastic modulus, affect said model, so that there arises phase displacement in the conveyance of the web such as to lead to a poor alignment of the pattern at the moment of application of the component.
In order to control the phase of the web, it is in general known to provide reference marks on the web, which have the purpose of enabling identification of the individual stretches of web and detection of said reference marks by means of a purposely provided sensor. For example, the U.S. Pat. No. 5,766,389 describes a method for the production of disposable absorbent articles, which envisages use of a layer bearing components and a layer bearing reference marks. Said layers are designed to be assembled together, and it is envisaged to detect the distance between two successive reference marks in order to control the speed of the layer with reference marks. Similar methods are also known from the U.S. Pat. No. 5,930,139, U.S. Pat. No. 5,932,039, U.S. Pat. No. 5,964,970, and U.S. Pat. No. 6,033,502.
The above solutions do not enable, however, an optimal control of phase displacement that will be able to take into account the variations of the characteristics of extensibility of the treated web material.